A battery pack for an electric vehicle can include a number of battery units connected in series, each of which needs to be charged equally. A charger can charge the battery pack to a predetermined voltage per battery unit. However, the individual battery units may not be charged to the same level, and the discrepancy between the units' state of charge levels can cause safety, reliability, and performance problems. The battery pack capacity is limited to the capacity of the lowest battery unit. Further, when some battery units have higher state-of-charge levels than others, the overcharged units can lead to unsafe conditions, such as fire and explosion. Additionally, when some battery units have lower state-of-charge levels, as the battery discharges, those units may discharge to a level resulting in permanent loss of unit capacity.